The present invention relates to RF receivers, and more particularly to an agile FM demodulator that can be used for detecting data transmitted over a cable television network.
Cable television systems supply a plurality of carriers within a predefined bandwidth. The carriers include television program signals assigned to specific channels, digital data (typically on an FM data path) for use in controlling converter boxes supplied by the cable television company, and may also include separate audio or information channel signals. The traditional approach for detecting and demodulating individual carrier signals is to use a superheterodyne receiver. Such a receiver provides an intermediate frequency ("IF") filter to select a desired carrier and reject all others.
Phase lock loop ("PLL") circuits can be used to detect a signal on a channel. A complicated tuning system is required, however, to provide a frequency agile PLL demodulator. In such applications, a voltage controlled oscillator ("VCO") used in the PLL has to cover the entire desired frequency spectrum. Without a tracking input filter, the output of the phase detector might result in the VCO locking onto an undesired channel. This can happen even with a tracking filter if the interfering channel is substantially stronger than the desired one. Since the separation between carriers is small in television applications, it is difficult and expensive to manufacture suitable tracking filters for use with a PLL detector.
It would be advantageous to provide a PLL circuit for detecting channel signals without the need to provide a tracking input filter. The present invention provides such apparatus.